In processing machinery there are many functions involving the fine dispensing of liquids, such as onto workpieces, where the volume being dispensed must be carefully controlled. However, in many of these applications the apparatus for detecting the volume of liquid being dispensed is inaccurate and inefficient in controlling the correction of the process when the dispensed volume is not a preselected or target volume. This can lead to costly product waste and machine down time. For example, in the fabrication of semiconductor wafers, the wafer surfaces are coated with a glass that is dropped from a dispenser onto the wafer surface at its center while the wafer is being spun so that the glass is spread by centrifugal force evenly across the surface. As the SOG coating (dripping) amount is small, about 2 cc., and its density is about 0.86 gm/cc., its weight is only about 1.6-1.7 gms. This 2 cc. amount can be transported from a bottle or other reservoir and dispensed from a very small Teflon tubing, so that it is very difficult to effectively determine how much liquid has been dispensed and whether there is enough SOG dispensed onto the wafer. Presently, such processing machines use a level sensor, already in the machine as purchased, which senses the capacitance of the dielectric material (i.e., SOG). But, this sensor can only detect the SOG level, and essentially tells whether the SOG is all consumed. It can not be used to detect the volume of each dispensing of SOG, i.e., whether it was 2 cc. or not. Consequently, the machine may be dispensing an insufficient amount of liquid for proper coating over a considerable period resulting in the loss of an inordinate amount of product; or conversely, if the sensor indicates incorrect dispensing when the amount is acceptable, constant unnecessary machine stoppage can be costly.
There is clearly a need in the fine liquid dispensing art of a method and means for more accurately determining the amount of liquid being dispensed during a single application to a surface or workpiece.
It is therefore an object of the present invention to provide an apparatus and method for closely monitoring the amount of fine liquid being dispensed during a coating or similar process.
It is another object of the invention to provide an apparatus and method for accurately determining the volume of fine liquid being dispensed during each application in a process.
It is a further object of the invention to provide a system and method for accurately measuring the volume of fine liquid being dispensed during each application in a process and producing an alarm and machine interlock when the volume becomes insufficient.